1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic method.
2. Description of the Related Art
Conventionally, image forming apparatuses are known that employ an electrophotographic method, such as an electrophotographic copier, an electrophotographic printer (e.g., light-emitting diode (LED) printer, laser beam printer, etc.), and an electrophotographic facsimile machine.
The image forming apparatus of this type includes a charging device which uniformly charges a surface of an electrophotographic photosensitive member and an image exposure device which exposes the charged surface of the electrophotographic photosensitive member to light to form an electrostatic latent image. The electrophotographic photosensitive member is drum-shaped or endless-belt-shaped and can be rotated at a predetermined speed. The electrophotographic photosensitive member used in electrophotography is hereinafter referred to as a photosensitive drum or a drum.
The electrostatic latent image is formed into a toner image by a development device using developer (hereinafter referred to as toner). The toner image is transferred to a transfer material by a transfer device. Subsequently, the toner image is fixed onto the transfer material as a fixed image by a fixing device. Residual transfer toner remains on the surface of the photosensitive drum after the toner image is transferred to the transfer material. The residual transfer toner is removed by a cleaning device and then the photosensitive drum prepares for the next image forming operation.
In recent years, many image forming apparatuses use a charging device employing a contact charging method. Most of such charging devices use a conductive roller of a roller-charging-type as a contact charging member, to which a voltage is applied. The conductive roller of the roller-charging-type is classified into two types. One employs a direct current (DC) system which uses only a direct current voltage as a charging voltage. The other employs an alternate current (AC) superposition system which applies a voltage whose DC and AC components are superposed.
The AC superposition system is capable of uniformly charging a photosensitive drum surface. However, in the AC superposition system, electrical discharge occurs in a micro gap between the contact charging member and the photosensitive drum, which shortens a life of the photosensitive drum. This is because the discharge causes damage to the photosensitive drum surface and increases abrasion loss of the photosensitive drum.
On the other hand, the DC system discharges less energy in the micro gap between the contact charging member and the photosensitive drum. Thus, the damage to the photosensitive drum is a little and the life of the photosensitive drum can be increased compared to the AC superposition system. Accordingly, from a viewpoint of increasing the life of the photosensitive drum, the DC system is suitable for charging the photosensitive drum. However, there have been the following problems in charging the photosensitive drum particularly with the DC system.
After an image is formed, a surface potential of the photosensitive drum may not be uniform due to an image pattern that has been formed on the surface. If the next charging is performed in this state, the photosensitive drum surface may not be uniformly charged depending on the image which was formed in the preceding image forming. As a result, the surface potential of the photosensitive drum becomes non-uniform when the photosensitive drum surface is exposed to a laser beam by the image exposure device next time. More specifically, if the image pattern with a strong contrast is first formed and subsequently a half-tone image is formed, the preceding image pattern may appear in the half tone image, namely a “ghost image” is formed.
In order to prevent formation of the ghost image, the photosensitive drum is uniformly irradiated (i.e., whole surface exposure) by a pre-charge exposure device having a light source such as a LED to make the surface potential of the photosensitive drum uniform which was exposed by the image exposure device in the previous image forming. In other words, a light portion potential corresponding to a potential of an area which is exposed in the previous image forming and a dark portion potential corresponding to a potential of an area which is not exposed in the previous image forming are equalized. In this way, the photosensitive drum surface can be uniformly charged the next time and the formation of the ghost image can be prevented. For preventing the formation of the ghost image, it is effective to use the pre-charge exposure device which is capable of emitting a large quantity of light.
However, if the photosensitive drum is uniformly irradiated by the pre-charge exposure device and the surface potential of the photosensitive drum becomes completely uniform, a problem of scatter of the residual transfer toner arises. The residual transfer toner is the toner remaining on the photosensitive drum which is not transferred to the transfer material during a transfer process. The residual toner may scatter in an image forming apparatus which performs pre-charge exposure before a cleaning process. The toner image is formed on the electrostatic latent image which is formed on the photosensitive drum surface by the image exposure device. The residual transfer toner on the photosensitive drum which is not transferred to the transfer material during the transfer process is also left on the electrostatic latent image. If the potential of the electrostatic latent image is completely and entirely uniformed by the pre-charge exposure device, an adhesive force between the toner and the photosensitive drum will be reduced. Accordingly, the residual transfer toner will no longer stay on the surface of the photosensitive drum. As a result, the toner scattering occurs. From a viewpoint of preventing the toner scattering, a small quantity of light is effective with respect to the light emitted from the pre-charge exposure device.
Further, decrease in the light quantity of the pre-charge exposure device when the pre-charge exposure device is used for a long time is discussed in Japanese Patent Application Laid-Open No. 11-174755. If the light quantity of the pre-charge exposure device decreases, a vestige of the previously-formed image may not be successfully deleted and may result in the formation of ghost image.
Thus, in order to reduce the formation of ghost image and prevent the toner scattering at the same time, it is necessary to optimize the light quantity emitted from the pre-charge exposure device depending on variations in manufacturing and the change caused through device usage with respect to the pre-charge exposure device such as a LED, a photosensitive drum film thickness, and photosensitive drum sensitivity.
Conventionally, the light quantity of the pre-charge exposure device has been controlled according to a relation between the surface potential of the photosensitive drum detected by a surface potential detection device and a current applied to the charging device as discussed in Japanese Patent Application Laid-Open No. 11-174755. Since the surface potential detection device of the photosensitive drum can directly measure the surface potential of the photosensitive drum, the light quantity of the pre-charge exposure device can be controlled to obtain a desired surface potential of the photosensitive drum surface.
Further, Japanese Patent Application Laid-Open No. 2004-334063 discusses a technique for estimating the photosensitive drum film thickness (film thickness of photosensitive layer) by applying a detection bias to a charging member and measuring a current that flows through the charging member. Thus, the light quantity of the pre-charge exposure device can be changed according to the film thickness of the photosensitive drum. According to the technique, if the film thickness of the photosensitive drum is thick, the pre-charge exposure device adjusts the light quantity to a small value. When the film thickness of the photosensitive drum becomes thinner along with the use of the image forming apparatus, the light quantity is increased.
However, since the surface potential detection device is expensive, employment of the surface potential detection device is not desirable from the viewpoint of providing an inexpensive image forming apparatus.
Applying detection bias to the charging member to estimate the photosensitive drum film thickness from the current that flowed through the charging member, and adjusting the light quantity of the pre-charge exposure device according to the film thickness is effective to a certain extent with respect to variation in photosensitive drum film thickness and photosensitive characteristics. However, it is difficult to estimate a change in the light quantity of the pre-charge exposure device occurring due to variations in manufacturing and the wear caused through the long time use, and adjust the light quantity of the pre-charge exposure device. Therefore, to deal with the variations in the manufacturing, more precise adjustment of the light quantity of the pre-charge exposure device has been required before shipment. However, when the light quantity is precisely adjusted, manufacturing efficiency will be lowered which will result in increased cost. Further, if the light quantity of the pre-charge exposure device is reduced due to the long time use or stain on the pre-charge exposure device caused by a user, the ghost image is formed on the drum. Furthermore, individual variation of the contact charging member or variation in resistance value according to a use environment may cause a difference in charge current value which leads to insufficient detection accuracy.